Method of reducing arcing in sealed electrical controls and articles produced thereby



Dec. 5, 1961 J. L. CLELLAND 3,012,122 METHOD OF REDUCING ARCING INSEALED ELECTRICAL CONTROLS AND ARTICLES PRODUCED THEREBY Filed Aug. 18,1959 METHOD OF REDUCING ARCING IN SEALED ELECTRICAL CONTROLS ANDARTICLES PRO- DUCED THEREBY James L. Clelland, Hudson, Ohio, assignor toMechanical Industries Production Company, Akron, Ohio, a corporation ofOhio Filed Aug. 18, 1959, Ser. No. 834,542

6 Claims. (Cl. 200-138) The present invention relates to methods andmeans for the reduction of arcing in thermostats, motor protector, orother sealed electrical controls.

In the control of temperatures of electrical heating apparatus, andelectrical appliances in general, it is customary to employ temperaturecontrolling devices in which a bimetal strip changes its shape whenheated and cooled and such bimetal strip is used to open the electricalcircuit of the apparatus when the temperature reaches a desired limit.In most apparatus, such temperature controls are hermetically sealed andinclude a conductive capsule, or case, usually of copper. A bimetalstrip, insulated from the capsule, is mounted therein, and the capsuleand the free end of the bimetal strip are each provided with a contactbutton, usually of silver, which engage each other when the control iscold and complete a circuit, or which separate at a critical temperatureto break the circuit, or

vice versa.

It has been found that temperature controls or thermostats of the sealedtype sometimes break down under high voltage conditions and examinationindicates that the breakdown may have been due to arcing within the caseaccompanied by ionization of the air contained in the case or capsule,which actions would increase conductivity of the contained gases, andpermit further arcing. Such arcing appears to have occurred as burned orcharred portions of the bimetal strip and blackened areas of theinterior of the capsule have been found in many thermostats after theyhave been tested, or used until failure occurs. These burned or charredportions of the bimetal strip may occur at positions remote from thefree end of the bimetal strip where electrical currents normally wouldnever flow.

It is an object of the present invention to reduce arcing in sealedthermostats and other temperature, or electrical controls.

It is a further object of the invention to provide a novel method oftreating thermostats, and similar articles; to improve the service lifethereof.

The foregoing and other objects will appear from the followingdescription, reference being had to the accompanying drawing forming apart hereof.

Referring to the drawing, which shows a longitudinal cross-sectionalview of a temperature control or thermostat constructed in accordancewith and embodying the invention, the numeral 10 designates a bimetalstrip comprising a layer 11 of a metal having one coefiicient ofexpension and a layer 12 having a higher coefficient of expansion. Thestrip extends through a tubular body 13 of insulating materials such asa plastic impregnated fibergl'ass material. A seamless capsule, or case14, usually made of copper, surrounds the free end of the bimetal stripand is contracted about the bimetal strip and the insulation body 13 asat 15 to provide an air-tight seal.

Within the capsule 14, a non-corroding contact button I 16 of metal,such as silver, is welded to the metal capsule, and a similar contactbutton 17 is welded to the bimetal strip at its free, end to normallycontact the button 16 under slight pressure. A wire lead 18 is soldered,brazed or otherwise secured to the exposed end of the bimetal 1 stripesat 19 and a wire lead 20 is suitably securedto a,

it terminal strip 2 1 lying against or secured to the capsule ii t,"

To provide for reducing ionization and consequent continued arcingwithin the capsule, a very thin continuous coating 22 is provided on theentire interior surface of the capsule. The coating may be provided bybringing the inner surface of the capsule before its assembly with theother elements into contact with a halogen element, such as iodine, toan extent to form a thin film of infinitesimal thickness over the innersurface. The deposit may be formed by wetting the interior of thecapsule with either an aqueous or alcohol solution of iodine. Thusfilling the inside of the case or capsule 14 with about a one to fourpercent water or alcohol solution of iodine for about between three tofive minutes, preferably with some agitation of the solution, and thenremoving the solution and drying gives a desirable, very thin film onthe inner surface of the case or capsule. The action provides anoticeable discoloration of the case surface, which discoloration is adarkening of the normal shiny surface of the copper capsule. Thecontact, or contact button 16 normally is secured to the capsule 14after it has been treated with the iodine solution. I am of the opinionthat the iodine combines with the copper of the capsule to form adeposit of copper iodide. The bimetal strip also may be treated with theiodine solution before the contact button 17 is secured thereto. Testsof thermostats of similar construction in which a number of thermostatshaving the coating and an equal number of similar thermostats not havingthe coating were compared showed that 1 less breakdown occurred in thethermostats having the coating, and that improved contact life wasobtained. If desired, the thermostat capsule would be filled with theiodine solution and the bimetal strip 19 and contacts 16 r and 17 may becovered with iodine, or compounds thereof. Any such iodine on thecontacts is removed by the initial current are therebetween. In someinstances, the bimetal strip 10 or the capsule 14 only may be coated, asthe iodine seems to distribute itself over all exposed surfaces in theclosed article with use thereof. I have observed that the destructivearcing previously mentioned only occurs when the sealing of thethermostat capsule approaches the hermetic, e.g. open ended or partiallysealed units do not exhibit the condition. Analysis of black depositsaround this silver contact yields copper and some iron but no silver. Iam of the opinion, therefore, that atmospheric ionization takes placewhen the high temperature electric arc breaks down the existing carbondioxide to form carbon monoxide. Carbon monoxide then reacts with copperof this case and iron of the bimetal in the presence of the silvercontacts as a catalyst to form very complex compounds of chainconstruction, most probably carbonyls. The resulting vapors being highlyconductive permit arcing from the bimetal to the case. While I am unableto identify the reaction, or lack of reaction, which takes place whenthe iodide coating is present, I am certain that there is some poisoningprocess of opposite complex nature to the formation of conductivecarbonyls, resulting in the elimination of internal arcing and greatlyimproved contact life in tightly sealed thermostat capsules. I

Although I prefer to apply the coating by wetting the surface of thecopper with an aqueous or alcoholic solution of elemental iodine, it isapparent that iodine might be applied by sublimation or other method asthe high temperatures of the thermostat arcs at make and break contactthereof, it is thought,'would distribute iodine particles over theexposed walls of the control article. While will be apparent to thoseskilled in the art that other em- ;bodiments of the invention may bemade without departing'from the invention as it is defined by thefollowing laims.

3,012,122 Patented Dec. 5, 19 61 What is claimed is:

1. A closed thermostat comprising a copper capsule, a bimetal stripextending into the capsule, insulating means between a fixed end of thestrip and the wall of said cap sule and sealed thereto, said bimetalstrip and said capsule having contacts normally engaging to close acircuit and adapted to open the circuit upon rise in temperature, and amaterial within said capsule adapted to reduce ionization due to arcing,said material comprising a continuous thin iodine-containing coatingcovering the entire inner surface of the copper capsule.

2. A closed thermostat comprising a copper capsule, a bimetal stripextending into the capsule, insulating means between a fixed end of thestrip and the wall of said capsule and sealed thereto, said bimetalstrip and said capsule having contacts normally engaging to close acircuit and adapted to open the circuit upon rise in temperature, and amaterial within said capsule adapted to reduce ionization due to arcing,said material comprising a continuous thin coating of iodine-containingmaterial integrally bonded to the inner surface of the copper capsule.

3. A closed thermostat comprising a copper capsule, a bimetal stripextending into the capsule, insulating means between a fixed end of thestrip and the wall of said capsule and sealed thereto, said bimetalstrip and said capsule having contacts normally engaging to close acircuit and adapted to open the circuit upon rise in temperature, and amaterial within said capsule adapted to reduce ionization due to arcing,said material comprising copper iodide.

4. The method of reducing arcing in a closed case electrical controlarticle which comprises providing a coating over the interior of acoppercase of the article by contact of said case for about 3 to 5 minutes toa l to 4 percent solution of iodine, drying the solution to deposit acoating of iodine onto the entire interior surface of said case, andcompleting the article including sealing the case.

5. A method as in claim 4 where any components to be assembled in saidcase are provided with a coating of a References Cited in the file ofthis patent UNITED STATES PATENTS 1,819,198 Reichmann et al Aug. 18,1931 1,862,317 Ringwald Dec. 23, 1927 2,497,397 Dales Ian. 19, 19462,757,262 Yearnans May 23, 1955 2,773,962 Perst Oct. 10, 1955 2,925,599Wells July 29, 1958

